The present invention generally relates to processes for removing acidic gases from flue gases, such as the exhaust gases produced by coal and oil-fired utility and industrial plants.
Sulfuric acid mist can be a significant problem for coal or oil-fired power plants by causing corrosion inside the system as well as creating environmental concerns from emissions that exit the system. Gas-liquid contactors and absorbers, or scrubbers, are widely employed to remove sulfur dioxide (SO2), hydrochloric acid (HCl), hydrofluoric acid (HF) and, to a much lesser extent, sulfur trioxide (SO3) and/or sulfuric acid (H2SO4), from flue gases produced by utility and industrial plants. Scrubbers generally have a quench zone where a liquid media is brought into intimate contact with a flue gas to remove acidic gases by absorption. The process by which acidic gases are removed from flue gases in this manner is generally referred to as wet flue gas desulfurization (wet FGD).
Sulfur dioxide is typically present in flue gases produced by coal and oil-fired boilers at much higher concentrations than HCl, HF and SO3. Removal of SO3 and sulfuric acid vapors from such flue gases helps to reduce a visible plume produced as a result of the formation of a sulfuric acid mist in the quench zone of FGD systems. The average particle size of such a mist is generally in the submicron range, which is sufficiently small to enable the mist to penetrate most FGD scrubbers. Sulfuric acid emissions of as little as about 5 ppmv will often result in a visible plume. Therefore, it is desirable to remove SO3 and H2SO4 from flue gases upstream of the FGD system. During the combustion of coal, most chlorides present in the coal are converted to HCl. The HCl in flue gases is removed very efficiently by SO2 removal systems, and as a consequence can become highly concentrated in the scrubbing solutions. High concentrations of chlorides can interfere with the scrubber efficiency and lead to disposal problems. Therefore, the removal of HCl prior to the FGD system can also be beneficial in certain cases.
As a solution to the above, U.S. Pat. No. 6,126,910 to Wilhelm et al. and U.S. Pat. No. 6,803,025 to Meserole et al., incorporated herein by reference, teach the use of soluble sulfite/bisulfite solutions, such as sodium sulfite (Na2SO3), sodium bisulfite (NaHSO3), potassium sulfite (K2SO3.2HOH), potassium bisulfite (KHSO3) and mixtures thereof to remove SO3 and other acidic gases from a flue gas without removing or decreasing the amount of sulfur dioxide also present in the flue gas. The process entails injecting (e.g., spraying) a concentrated solution containing a sulfite/bisulfite into the flue gas stream to react acidic gases (e.g., HCl, HF and/or SO3) and form a reaction product, without reacting the sulfur dioxide. After removal of the acidic gas(es), sulfur dioxide can be removed from the flue gas farther downstream using conventional scrubbing techniques, which can be rendered more technically and/or economically desirable as a result of the absence of SO3. According to Wilhelm et al., a soluble bisulfite salt such as sodium bisulfite selectively removes acidic gases such as HCl, HF and SO3, but will not remove sulfur dioxide. Wilhelm et al. teach that sulfur dioxide can be removed with solid reagents such as sodium carbonate (Na2CO3) and lime (CaO). Meserole et al. teach that soluble carbonate salts and soluble bicarbonate salts, if injected as a fine mist, can react with SO2 and upon drying form solid sulfite salts that react with SO3 and H2SO4 and reform SO2.